The present invention relates to field emission cathodes and, more particularly, to such a cathode of bio-molecular or eutectic composite microstructure capable of macroscopic beam current densities of J.gtoreq.100 A/cm.sup.2 without the formation of a surface plasma.
The need for high current density, high brightness, survivable cathodes is driven by several applications of interest, including microwave sources (C. W. Roberson, Proc. Soc. Photo-OOpt. Int. Eng. 453, 320 (1983), D. A. Kirkpatrick, G. Bekefi, A. C. DiRienzo, H. P. Freund, and A. K. Ganguly, Phys. Fluids B 1, 1511 (1989)) radars, communications, and ECRH heating of fusion plasmas, high power fast switches ("Vacuum Arcs, Theory and Applications," J. M. Lafferty ed., John Wiley & Sons (New York, 1980)), high gradient accelerators (J. LeDuff, Proc. Lin. Acc. Conf., 285, (Newport News, VA, 1988)), and electron beam processing of materials ("The New York Times," pp. D1, Jun. 20, 1990). The properties that constitute the ideal electron source vary from application to application. For some, the capability to operate for long pulses at high current density is the most significant improvement. For others, it is not so much the highest current densities that would be available, as it is a combination of a modest increase in available current density combined with an increase in operational lifetime.
The existing technology for electron beam sources can be broken down into four groups: (1) thermionic cathodes, (2) laser driven photo-cathodes, (3) classical field emission cathodes, and (4) exploding or plasma field emission cathodes. The existing state-of-the-art can be summarized as follows:
1. Thermionic cathodes use a thermally activated, low work function material to act as an electron beam source. Older technologies simply used a barium-oxide coating that was painted on the desired emission surface. New technologies use a porous dispenser matrix to gradually deliver a scandate compound to the cathode surface. Commercially available technology delivers 20 A/cm.sup.2 for lifetimes longer than 1000 hours, but requires vacuum pressures less than 10.sup.-7 Torr and has a long list of materials that will chemically poison the surface even at that level. Notably excluded materials include hydrocarbons, flourocarbons, and stainless steel. Research cathodes (R. E. Thomas. J. W. Gibson, G. A. Haas, and R. H. Abrams, IEEE Trans. Elec. Dev. 37, 850 (1990)) have produced cathode current densities as high as 140 A/cm.sup.2, but these require even higher vacuum standards and have problems with beam quality, reproducibility, and lifetime. All of these cathodes require a heater element to maintain the cathode surface at an elevated temperature anywhere between 900.degree. and 2200.degree. C. PA1 2. Laser driven photo-cathodes (R. L. Scheffield, E. R. Gray, and J. S. Fraser, Nuc. Instr. Meth. A272, 222 (1988), C. Sanford and N. C. MacDonald, J. Vac. Sci. Technol. B 6, 2005 (1988)) use an intense pulse of light to photo-eject electrons from a low work function (typically Cesiated) surface. They can produce very high instantaneous current densities (&gt;60 kA/cm.sup.2), albeit for very short times. They require very high vacuum, with vacuum pressures in the range of 10.sup.-9 Torr or less. Other approaches, using bare metals such as Copper, operate in slightly poorer vacuum (10.sup.-8 Torr) but have considerably poorer efficiency of conversion of the laser light. PA1 3. Vacuum field emission cathodes are typically tungsten fibers, and are predominantly used in Scanning Electron Microscopes (SEM). They produce an electron beam by classical Fowler-Nordheim quantum tunneling of electrons from near the Fermi level into the vacuum. The large electric field that is required is obtained from the very large field enhancements near a sharp point. Beam brightness is very high, since the beam is essentially produced by a point-emitter. The current density is very high, but this is a single-tip emitter, and therefore the total current is very low. Other researchers (C. A. Spindt, K. R. Shoulders, and L. N. Heynick, U.S. Pat. Nos. 3,755,704 (1973), and 3,812,559 (1974), C. A. Spindt, I. Brodie, L. Humphrey, and E. R. Westerberg, J. Appl. Phys. 47, 5248 (1976), C. A. Spindt, C. E. Holland, and R. D. Stowell, Appl. Surf. Sci, 16, 268 (1983), G. J. Campisi and H. F. Gray, Mat. Res. Soc. Symp. Proc. 76, 67 (1987), H. H. Busta, R. R. Shadduck, and W. J. Orvis, IEEE Trans. Elec. Dev. ED-36, 2679 (1989)) are pursuing the same objective using microlithographic approaches that produce "gated" arrays of small pyramids or cones. PA1 4. Exploding or plasma field emission cathodes (D. D. Hinshelwood, Naval Research Laboratory Memorandum Report No. 5492 (1985)) are the cornerstone of the extremely high pulsed power regime. Cathode current densities in excess of 1 MA/cm.sup.2 have been demonstrated. They tolerate moderate to poor vacuum quite well. The quality of the electron beam is not high, but high quality beams may be obtained by passing the electrons through an emittance filter. This may reduce the beam current to 1% of its initial value, but one still has a high current beam that is now also a high quality beam. The truly significant drawback to these cathodes is their inherent inability to operate for long pulses (&gt;1 .mu.sec) or at a high repetition rate (&gt;10 Hz).
The basic limitations for these four types of electron beam sources dictates which is used for a specific application. Most existing technology utilizes thermionic emitters. Some high power research experiments use plasma field emission cathodes because of their high instantaneous power capability and their ease of operation. Classical field emission cathodes are almost exclusively used in SEMs, and laser photo-cathodes are still mostly in a research phase.
The class of thermionic emitters is the dominant segment of the electron beam source pie because of its DC and long pulse capability. All radars, all RF sources that drive RF linacs, all conventional tubes, and almost all of the commercial electron source demand is currently utilizing thermionic cathodes. Almost any application that requires substantial average power capability must use a thermionic emitter.
Applications that require high quality, high current electron beams are also limited by the thermionic emitters. The normalized electron beam brightness is defined as ##EQU1## where J is the cathode current density in A/cm.sup.2, .gamma.=1+(eV/m.sub.o c.sup.2) is the relativistic factor, .beta.=v/c is the electron velocity normalized to the speed of light, and .delta..theta.=v.perp./v.parallel. is the FWHM in the transverse velocity spread angle of the electron beam distribution. The maximum beam brightness available from a thermionic emitter is theoretically on the order of 10.sup.8 A/cm.sup.2 -rad.sup.2. But achieving a high beam current density in addition to this beam brightness demands magnetic compression of the emitted beam. Creating very accurate magnetic fields over any substantial area is extremely difficult, and this limits the practically available brightness for a .about.1 A beam using a thermionic emitter to .about.10.sup.6 A/cm.sup.2 -rad.sup.2.
A device that is most significantly affected by electron beam brightness is the free-electron laser (FEL). For the FEL it is the brightness of the electron beam that determines the characteristics of the device. The minimum operational wavelength is determined by the electron beam emittance, .epsilon..sub.n =(1/.pi.).sqroot.I/B.sub.n, EQU .lambda..sub.rad &gt;.pi..epsilon..sub.n ( 2)
Given a desired operational wavelength, the electron beam brightness determines how much beam current can participate in the interaction. This in turn determines whether the interaction will be high gain, low gain, or no gain. Existing technology for thermionic emitters makes 10 .mu.m about the transition wavelength for the high gain to low gain regimes, and optical wavelengths in the blue the transition from the low gain to no gain regimes. Increasing the available beam brightness shifts these transition points to shorter wavelengths.
The other major category of electron sources, explosive or plasma field emission cathodes, is limited in pulse length to approximately 1 .mu.sec due to the inherent presence of an expanding cathode plasma. More particularly, field emission cathodes operate by applying a large electric field to an emission surface, perhaps reactor grade graphite (carbon). The large field draws electrons out of the materials by quantum tunneling. Presently, this process describes only the initial phase of "turn-on". The initial current generated in this phase is emitted from small microscopic protrusions in the surface of the material; the large currents drawn through these small tips results in large local Ohmic heating of the tips, which subsequently ablate and produce a cathode surface plasma. Subsequent emission of electrons occurs from this intermediate of the cathode plasma, which has a very low work function and allows for very high current densities to be generated (I&gt;100,000 A/cm.sup.2). The significant drawback of this process is that the generated cathode plasma typically expands towards the anode at a rate of 1-2 cm/.mu.sec, which limits the useful pulse length and precludes repetitively pulsed operation. More specifically, the expanding plasma reduces the effective cathode-to-anode distance because emission occurs from the leading edge of the plasma. The decreased cathode-anode spacing increases the current which is drawn, since this type of situation is described by Child-Langmuir space-charge limited flow, resulting in electron gun impedance collapse. A significant advantage to these cathodes is their relative insensitivity to the vacuum environment. They operate quite well in vacuum of 10.sup.-4 Torr, and do not poison. This makes them ideal electron beam sources for experimental apparatus that do not require long pulse or repetitively pulsed capability. They are also inexpensive and do not require any special handling.
In view of the foregoing discussion it should be apparent that there is a continuing need for an electron beam source which combines the advantages of the four groups as described and which reduces to a minimum the stated shortcomings or disadvantages. The present invention satisfies such a need by providing a field emission cathode which combines the advantages of both thermionic and plasma field emission cathodes. Like the thermionic cathodes, they have the capability to operate DC or repetitively pulsed. Like the plasma field emission cathodes, they are inexpensive, require minimal care in handling, operate well in moderate vacuum, and do not poison. They do not require a heater, nor its associated power supply. Cathode current densities J&gt;200 A/cm.sup.2 and brightness B.sub.n &gt;10.sup.7 A/cm.sup.2 -rad.sup.2 are possible.